Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise an illumination source, image gathering optics, focal plane array imaging detectors and imaging spectrometers.
In general, the sample size determines the choice of image gathering optic. For example, a microscope is typically employed for the analysis of sub micron to millimeter spatial dimension samples. For larger targets, in the range of millimeter to meter dimensions, macro lens optics are appropriate. For samples located within relatively inaccessible environments, flexible fiberscope or rigid borescopes can be employed. For very large scale targets, such as planetary targets, telescopes are appropriate image gathering optics.
For detection of images formed by the various optical systems, two-dimensional, imaging focal plane array (FPA) detectors are typically employed. The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge-coupled device (CCD) detectors or CMOS detectors are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems.
Spectroscopic imaging of a sample can be implemented by one of two methods. First, a point-source illumination can be provided on the sample to measure the spectra at each point of the illuminated area. Second, spectra can be collected over the entire area encompassing the sample simultaneously using an electronically tunable optical imaging filter such as an acousto-optic tunable filter (AOTF) or a LCTF. This may be referred to as “wide-field imaging”. Here, the organic material in such optical filters are actively aligned by applied voltages to produce the desired bandpass and transmission function. The spectra obtained for each pixel of such an image thereby forms a complex data set referred to as a hyperspectral image (HSI) which contains the intensity values at numerous wavelengths or the wavelength dependence of each pixel element in this image.
Spectroscopic devices operate over a range of wavelengths due to the operation ranges of the detectors or tunable filters possible. This enables analysis in the Ultraviolet (UV), visible (VIS), near infrared (NIR), short-wave infrared (SWIR), mid infrared (MIR), long wave infrared (LWIR) wavelengths and to some overlapping ranges. These correspond to wavelengths of about 180-380 nm (UV), 380-700 nm (VIS), 700-2500 nm (NIR), 850-1800 nm (SWIR), 2500-25000 nm (MIR), and 7500-13500 nm (LWIR).
Proliferation of chemical, biological, and explosive (CBE) threats is a growing danger to civilian and military personnel. There exists a need for sensor systems that can rapidly detect these CBE threats at a standoff distance. Examples of technologies that hold potential for such detection include short wave infrared (SWIR) spectroscopy and Raman spectroscopy.
Raman spectroscopy requires probing a sample with a laser beam. The issue of laser safety must be solved before widespread deployment is possible. There exists a need to develop strategies to eliminate the laser hazard to both operators and bystanders.